In conventional fluid bed processing equipment 10 of the foregoing type, as depicted in FIG. 1, one or more filter elements 11 are used to separate and collect airborne particles discharging from a process vessel 12 into an exhaust plenum 15. Air initially latent with particles may only flow from the process vessel 12 to the exhaust plenum 15 through the filter element 11. Over time the particulate accumulates on the filter element 11 restricting the flow of air from the process vessel to the exhaust plenum preventing effective processing. Periodically the filter must be cleaned by knocking the particulate accumulation off of the retaining filter element 11. Commonly a nozzle 16 is arranged at the point where the filter element 11 terminates into the exhaust plenum 15. The inlet end of the nozzle 16 is connected to a high compressed air source 18 that is controlled by a quick acting valve. When the valve is momentarily actuated, a pulse of air is released from the compressed air source 18 and travels down the nozzle 16 toward the retaining filter 11. The burst of compressed air temporarily reverses the flow of air from the process vessel 12 to the exhaust plenum 15 and redirects the air so that it then moves from the compressed air source into the process vessel. A venturi effect 19 occurs if the nozzle 16 is properly positioned for also drawing air from the exhaust plenum into the process vessel along with the compressed air. This sudden pulse of air acting in the reverse direction serves to dislodge the accumulated particulate from the face of the filter 11 on the process side. The particulate drops off of the retaining filter 11 and returns to the process vessel 12 for further processing or collection.
In many instances, the process occurring does not allow the flow of air to be stopped from the process vessel to the exhaust plenum. This hampers the cleaning and often makes the reverse pulse method of filter cleaning ineffective. This can occur for several reasons, but is normally attributed to the size the particles being cleaned, the frequency by which the filter can be cleaned, improper positioning and sizing of the compressed air nozzle, and an excessive air velocity across the filter element. When this occurs, a large differential pressure can be created between the process vessel and the exhaust plenum. This large pressure differential hinders the venturi effect that draws air from the exhaust plenum and reduces the effectiveness of the air pulse. Rather than dislodging the accumulated particulate from the retaining filter, the air takes the path of least resistance with most of the air traveling into the exhaust plenum and only a small amount traveling into the process vessel with a force too weak to dislodge accumulated material from the filter.
Attempts to overcome the foregoing problems have included increasing the frequency of cleanings, increasing the pulse duration, increasing the pulse force by increasing the pressure of compressed air source, increasing the number of retaining filter elements, or combining this method with mechanical filter cleaning. Often these methods are impractical to implement and reduce efficiency of the process.